The invention relates to a process and a device for copying or enlarging photographic color images in accordance with the subtractive method. In the copying of photographic color images in accordance with the subtractive method, filtering devices are often inserted which help influence the color composition of the copying light in that three color filters, adapted to the individual sensitivity ranges of the copying paper, may be inserted into a beam of light emanating from a white-emitting light source. The filters inhibit the beam portion corresponding to their particular blocking area, depending on the depth of their insertion into the light beam. The better the sensitivity range of the individual color layers corresponds with the particular color filter, the greater the blocking action achieved for the copying paper. However, since individual sensitivity ranges of the copying paper overlap, the blocking ranges of the corresponding color filters must also overlap each other if the greatest possible filter densities are to be achieved for the individual sensitivity ranges. This results in that with a change of the insertion depth of the color filters into the beam of light, not only the illumination of the desired sensitivity range but also of the adjacent one is influenced to a certain extent. The degree of this two-way influence depends on the expansion of the overlapping areas and on the adaptation of the color filters to the particular sensitivity range. This two-way influence makes the determination of the correct color balance more difficult and has a disturbing effect on any subtractive copying method, be it that the correct color balance is produced by repeated copying attempts or determined by measurements before or during the copying procedure.
In the previously known filter devices, the filters pertaining to the overlapping sensitivity ranges of the copying paper are usually arranged in such a manner that blocking edges adjacent thereto proceed through the point of intersection of the spectrum-sensitivity curves defining the adjacent sensitivity ranges.
By this step, a compromise is reached between the limitation of the disruptive, reciprocal influence on the one hand and the achievable filter density in the basic colors corresponding to the individual sensitivity ranges on the other hand.
The disruptive effect by the reciprocal influence of sensitivity ranges, adjacent to each other, may be produced in such a manner that the individual color filters, aside from their main density in their basic colors, also have a secondary density in the adjacent basic color, the degree of this secondary density depending on the course of the blocking edge of the color filter facing the adjacent basic color. These secondary densities may be decreased in that the course of the blocking edges of the particular color filters is determined in such a manner that their blocking range only coincides with a small part of the overlapping area of the sensitivity ranges. In such a case, however, an undesirable decrease of the achievable main density in the respective basic colors must be taken into consideration.
In the known filter devices, wherein color filters may be inserted into a beam of light emanating from a white-emitting light source, each movement of the blocking edges proceeding through the overlapping range is expressed as a change of the blocking action of the corresponding color filters, inconsistantcies therein being caused by temperature changes, for instance. This causes the drawback in such filter devices that a once-adjusted proportion of the basic color portions of the copying light is not stable and thus brings about changes in copying results which are difficult to control.
In mass production of such known filter arrangements, color filters with a very narrow toleration blocking edge course must be employed, since the inconsistancies caused by copy straying otherwise effect intolerable quality differences in the end product. Maintaining such narrow tolerance limits also disadvantageously affects production costs of the product.
A further drawback of the known filter arrangements consists of the blocking characteristic of the filters employed in many instances not proceeding sufficiently steeply and thus cannot be adjusted precisely enough to the sensitivity curve of the corresponding sensitivity range of the copying paper, something which limits the achievable filter density.
The object of the invention is to provide a method and a device for the copying of photographic color images in accordance with the subtractive method, wherein a filtering, independent of the adjacent basic colors, is possible and wherein the drawbacks, caused by the blocking characteristic of color filters or by their changes or straying, do not occur.